There are a few possible explanations, and it depends on how often you lose matches against opponents of 20%- or more.

If you had 1 or 2 max per season, it is generally called a troll. That means the engine favours your opponent from the outset. As an active manager, you would need to try to change whatever you could in-game to influence the game calculation to your favour, such as:
1) formation
2) tactics and orders
3) substitutions
4) set piece takers
5) bonus type

There is no guarantee all the changes could reverse the result, but it is very important to handle and try switching things around whenever we feel a troll game is under way, as it could easily happen in a CL or Cup 2nd leg, or even in both legs, or in the worst case a final.

If it happens more than a few times a season, then it is more likely to do with your team formation, tactics and orders. You may need to fine-tune some common counter formations and strategy according to your opponent’s.